ABSTRACT To minimize the adverse dosimetric effect caused by tumor motion, it is desirable to have real-time knowledge of the tumor position throughout the beam delivery process. A promising technique to realize the real-time image guided scheme in external beam radiation therapy is through the combined use of MV and onboard kV beam imaging. The success of this MV-kV triangulation approach for fixed-gantry radiation therapy has been demonstrated. With the increasing acceptance of modern arc radiotherapy in the clinics, a timely and clinically important question is whether the image guidance strategy can be extended to arc therapy to provide the urgently needed real-time tumor motion information. While conceptually feasible, there are a number of theoretical and practical issues specific to the arc delivery that need to be resolved before clinical implementation. The purpose of this work is to establish a robust procedure of system calibration for combined MV and kV imaging for internal marker tracking during arc delivery and to demonstrate the feasibility and accuracy of the technique. A commercially available LINAC equipped with an onboard kV imager and electronic portal imaging device (EPID) was used for the study. A custom built phantom with multiple ball bearings was used to calibrate the stereoscopic MV-kV imaging system to provide the transformation parameters from imaging pixels to 3D world coordinates. The accuracy of the fiducial tracking system was examined using a 4D motion phantom capable of moving in accordance with a pre-programmed trajectory. Overall, spatial accuracy of MV-kV fiducial tracking during the arc delivery process for normal adult breathing amplitude and period was found to be better than 1 mm. For fast motion, the results depended on the imaging frame rates. The RMS error ranged from approximately 0.5 mm for the normal adult breathing pattern to approximately 1.5 mm for more extreme cases with a low imaging frame rate of 3.4 Hz. In general, highly accurate real-time tracking of implanted markers using hybrid MV-kV imaging is achievable and the technique should be useful to improve the beam targeting accuracy of arc therapy.

[Show abstract][Hide abstract]ABSTRACT: The presence of metals in patient causes streaking artifacts in X-ray CT
and has long been recognized as a problem that limits various
applications of CT imaging. Accurate localization of metals in CT images
is a critical step for metal artifacts reduction in CT imaging and many
practical applications of CT images. The purpose of this work is to
develop a method of auto-determination of the shape and location of
metallic object(s) in the image space. The proposed method is based on
the fact that when a metal object is present in a patient, a CT image
can be divided into two prominent components: high density metal and low
density normal tissues. This prior knowledge is incorporated into an
objective function as the regularization term whose role is to encourage
the solution to take a form of two intensity levels. The function is
minimized by using a Gauss-Seidel iterative algorithm. A computer
simulation study and four experimental studies are performed to evaluate
the proposed approach. Both simulation and experimental studies show
that the presented algorithm works well even in the presence of
complicated shaped metal objects. For a hexagonally shaped metal
embedded in a water phantom, for example, it is found that the accuracy
of metal reconstruction is within submillimeter. The algorithm is of
practical importance for imaging patients with implanted metals.

[Show abstract][Hide abstract]ABSTRACT: Conformal radiation of moving target volumes is a challenging task in radiotherapy. The uncertainty about the actual location
forces the definition of large security margins, especially for target volumes in the lung or near diaphragm. Gating is one
strategy to overcome these limitations but needs a reliable estimation about the target position. Currently most approaches
try to estimate the position by recording external signals like changes of the circumference of the belly. Since there is
no clear relationship between those external signals and the position of the tumor, we try to establish an image guided approach
where the moving tissue is observed by fluoroscopic imaging during treatment. We demonstrate the prototype of the system which
can be used to assure correct patient setup and to observe organ motion. To limit additional dose for imaging the imaging
system is combined with a conventionally acquired external breathing signal which triggers image acquisition and latter one
controls the gating window. Since image processing is more complex, the challenge is to perform necessary analysis in a sufficiently
short time. First experiments have already shown that this task could be done in some few ms, if an analysis can be restricted
to a reasonable part (256*256) of the acquired images (1024*1024). The developed system integrates all essential adaptive
processes to establish a real time respiratory gated radiotherapy combining an external respiratory signal with an image-based
approach.

[Show abstract][Hide abstract]ABSTRACT: The Vero4DRT system has the capability for dynamic tumor-tracking (DTT) stereotactic irradiation using a unique gimbaled x-ray head. The purposes of this study were to develop DTT conformal arc irradiation and to estimate its geometric and dosimetric accuracy.
The gimbaled x-ray head, supported on an O-ring gantry, was moved in the pan and tilt directions during O-ring gantry rotation. To evaluate the mechanical accuracy, the gimbaled x-ray head was moved during the gantry rotating according to input command signals without a target tracking, and a machine log analysis was performed. The difference between a command and a measured position was calculated as mechanical error. To evaluate beam-positioning accuracy, a moving phantom, which had a steel ball fixed at the center, was driven based on a sinusoidal wave (amplitude [A]: 20 mm, time period [T]: 4 s), a patient breathing motion with a regular pattern (A: 16 mm, average T: 4.5 s), and an irregular pattern (A: 7.2-23.0 mm, T: 2.3-10.0 s), and irradiated with DTT during gantry rotation. The beam-positioning error was evaluated as the difference between the centroid position of the irradiated field and the steel ball on images from an electronic portal imaging device. For dosimetric accuracy, dose distributions in static and moving targets were evaluated with DTT conformal arc irradiation.
The root mean squares (RMSs) of the mechanical error were up to 0.11 mm for pan motion and up to 0.14 mm for tilt motion. The RMSs of the beam-positioning error were within 0.23 mm for each pattern. The dose distribution in a moving phantom with tracking arc irradiation was in good agreement with that in static conditions.
The gimbal positional accuracy was not degraded by gantry motion. As in the case of a fixed port, the Vero4DRT system showed adequate accuracy of DTT conformal arc irradiation.

Data provided are for informational purposes only. Although carefully collected, accuracy cannot be guaranteed. The impact factor represents a rough estimation of the journal's impact factor and does not reflect the actual current impact factor. Publisher conditions are provided by RoMEO. Differing provisions from the publisher's actual policy or licence agreement may be applicable.